Coherent population trapping, first observed in sodium atoms, is a manifestation of laser interaction with a three level system, where a coherence between two hyperfine ground state levels is achieved through a two photon process. In a three-level lambda (Λ) system, a particular laser excitation can provide a resonance condition in which destructive interference occurs between excitation pathways. The population is reduced in the upper state and trapped within the two ground states, creating a “dark state.” This reduction of fluorescence and absorption has been given the name “dark line.”
The transition between ground states of an alkali metal is electric dipole forbidden and has a long lifetime. The narrow resonance feature of this effect has stimulated the idea of an all-optical frequency standard. Introducing a buffer gas in a cell as well as using a cell with Paraffin-coated walls can reduce the linewidth. The dependence of the width and contrast of this narrow line on the laser power, the light polarization, the magnetic field, and the cell temperature of rubidium has been studied. In those experiments, a pair of phase locked lasers or a single laser source modulated by half of the ground-state hyperfine splitting has been used to create coherence between hyperfine split ground state levels. The coherence effect using a pulse train by adjusting its repetition rate to a submultiple of the hyperfine splitting of sodium has been observed. Pulse generation by current modulation of a laser diode has been used. Further advances in laser systems and laser spectroscopy are needed.